The present invention relates to photocatalytic processes and, more particularly, to a reactor and a method for the photopromoted catalyzed degradation of compounds in a fluid stream.
Considerable effort has been expended in recent years toward the development of methods to remove environmentally detrimental compounds from fluid streams. A particularly promising approach is the photopromoted catalyzed degradation of such compounds, as disclosed in Lichtin et al., U.S. Pat. Nos. 4,861,484 and 4,980,040, and Raupp et al., U.S. Pat. No. 5,045,288. Specific reactor structures are disclosed in Matthews et al., J. Phys. Chem. 1987, 91, 3328-3333; Robertson et al., U.S. Pat. Nos. 4,892,712, 4,966,759 and 5,032,241; and Anderson et al., U.S. Pat. No. 5,035,784. The specifications of the listed patents and the disclosure of Matthews et al. are incorporated by reference herein for all purposes.
In each of the foregoing systems, a catalyst is distributed uniformly within a reaction chamber. As a result, the intensity of light impinging on the catalyst is a function of distance from the light source. Intensity is very high close to the light source and falls off rapidly with increasing distance due to dispersion and absorption effects. As a result, one portion of the catalyst is exposed to light of extremely high intensity, while another portion is exposed to very little light. The difference in intensity is particularly extreme when high intensity light sources are used to increase the overall efficiency of commercial scale systems.
Unfortunately, the efficiency of photon utilization in photopromoted catalytic processes depends heavily on intensity. Efficiency is generally high at lower intensities and falls off rapidly as intensity increases above that of sunlight (approximately 10.sup.24 photons of actinic light per square meter per hour). Although not bound by any theory, a plausible explanation for this in the context of photooxidation is that the oxidation rate for attack by holes produced by illumination is limited by the transport of the oxidizable material and its oxidized form to and from the solid catalyst. This transport is a diffusion limited process which cannot keep up with the increase in hole production created by increased intensity. The excess holes tend to recombine with available electrons, a photon-wasteful process. Thus, the quantum efficiency of light catalyzed oxidation processes, which is essentially constant at illumination levels equivalent to or less than that of sunlight, varies as the square root of intensity at illumination levels above that of sunlight and varies as the reciprocal of intensity at very high illumination levels.
Accordingly, there is a need for a reactor and a process capable of more efficiently utilizing light in photopromoted catalytic processes.